1. Field of the Invention
This invention relates to a disc playback apparatus which uses a digital disc on which two-channel digital audio signals are recorded, and which memorizes digital data other than these two-channel digital audio signals.
2. Description of the Prior Art
A system using an optical digital audio disc (referred to as a compact disc) is a disc system which can reproduce high quality stereophonic musical sounds. If digital data such as data representative of characters, display data, program data, etc. other than stereophonic musical sounds can be reproduced by this disc system without significantly modifying the arrangement of the player, it will be possible to realize a playback apparatus for reproducing visual information such as charts and statistics by graphics, pictorial illustration by still pictures, and a video game apparatus by adding a display unit, thereby providing a wide applicable range for a compact disc system. The data memory capacity of the current compact disc is about 500M bytes, and therefore, a compact disc has a very great advantage as compared with the memory capacity of a standard flexible disc.
On the other hand, compact discs are principally utilized for reproduction of audio signals, thus, the beginning of the data on the disc is searched on a relatively large unit basis such as music program unit or phrase unit. However, as a memory device, the data have to be read out on a smaller unit basis in the order of 128 bytes to 10K bytes. For searching of the beginning of the data, a sub-coding signal is recorded on the compact disc together with the digital audio data. The reproduced digital audio data is written in a RAM (Random Access Memory) synchronously with a reproduced bit clock signal, and read out from the RAM by a reference clock signal. It is necessary for error correction, error interpolation and elimination of variation of time base (hereafter called jitter) to write the reproduced digital audio data in the RAM. The sub-coding signal is different from the digital audio data in that a complicated error correction code therefor is not processed or written in the RAM. Therefore, the reproduced sub-coding signal includes a jitter component. Because of that, a time lag exists between the reproduced main channel digital audio data and the reproduced sub-coding signal, and this time lag is not constant and always fluctuates.
Because of the time lag, even if the main digital data recorded on a compact disc is accessed with the use of the sub-coding signal, there is the possibility that a desired data block will not be accurately read out.